This invention relates to electrostatographic copying and more particularly to a novel method of imaging a particular type of electrostatographic photoreceptor. The art of xerography, as originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves the formation of an electrostatic latent image on the surface of a photosensitive plate normally referred to as a photoreceptor. The photoreceptor comprises a conductive substrate having on its surface a layer of a photoconductive insulating material. Normally, there is a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection from the substrate into the photoconductive layer upon charging of the plate's surface.
In operation, the plate is charged in the dark, such as by exposing it to a cloud of corona ions, and imaged by exposing it to a light/shadow image to selectively discharge the photoreceptor and leave a latent image corresponding to the shadow areas. The latent electrostatic image is developed by contacting the plate's surface with an electroscopic marking material known as toner which will adhere to those portions of the plate which retain the electrostatic charge.
One type of electrostatographic photoreceptor comprises a conductive substrate having a layer of photoconductive material on its surface which is overcoated with a layer of an insulating organic resin. Various methods of imaging this type of photoreceptor are disclosed by Mark in his article appearing in Photographic Science and Engineering, Vol. 18, No. 3, May/June 1974. The processes referred to by Mark as the Katsuragawa and Canon processes can basically be divided into four steps. The first is to charge the insulating overcoating. This is normally accomplished by exposing it to d.c. corona of a polarity opposite to that of the majority charge carrier. When applying a positive charge to the surface of the insulating layer, as in the case where an n-type photoconductor is employed, a negative charge is induced in the conductive substrate, injected into the photoconductor and transported to and trapped at the insulating layer-photoconductive layer interface resulting in an initial potential being solely across the insulating layer. The charged plate is then exposed to a light and shadow pattern while simultaneously applying to its surface an electronic field of either alternating current (Canon) or direct current of polarity opposite that of the initial electrostatic charge (Katsuragawa). This step is carried out until the plate's surface potential is driven to zero (Canon) or to a chosen potential opposite in sign to that of the original surface potential (Katsuragawa). The plate is then uniformly exposed to activating radiation to produce a developable image with potential across the insulating overcoating and simultaneously reduce the potential across the photoconductive layer to zero.
The technique of applying an electronic field to the surface of the photosensitive device simultaneous with imagewise exposure is not particularly adaptable to imaging in the full frame flash exposure mode. This is the case because the flash exposure of a full frame is generally on the order of about 50 microseconds; a period which is too short for corotrons of ordinary efficiency to drive the surface potential to the selected level. It has more recently been discovered that good contrast potentials can be achieved on an insulator overcoated plate with imaging in the full frame flash exposure mode by shunting the photosensitive device to some preselected voltage both before and after the imaging step. This process is effective in that it provides good contrast potentials. However, it is difficult to provide equal shunting to all parts of the segment to be imaged both before and after imagewise exposure. Since lead to trail edge uniformity is desirable in a copying process, an improvement in the previously described process would be desirable. Accordingly, it is an object of the present invention to provide a novel method for the formation of an electrostatic latent image on an insulator overcoated electrostatographic photosensitive device.
A further object is to provide such a method which is suitable for use in conjunction with imaging in the full frame flesh exposure mode.
An additional object is to provide such a method which provides lead edge to trail edge uniformity in contrast potentials.